High-output laser light sources are attracting attentions as light sources used in laser processing systems or laser displays. In an infrared region, solid-state lasers such as YAG lasers, fiber lasers using fibers doped with rare earths such as Yb and Nd and the like are being developed. In red and blue regions, semiconductor lasers using GaAs, gallium nitride, etc. are being developed and higher outputs are also being studied.
On the other hand, in a green region, it remains still difficult to directly generate green light from a semiconductor and it is a general practice to generate green light by wavelength-converting infrared light generated from the aforementioned solid-state laser or fiber laser by means of nonlinear optical crystal. Before the development of gallium nitride, there exists virtually no method for obtaining lights from a visible region to an ultraviolet region other than wavelength conversion using a nonlinear optical crystal. Various nonlinear optical materials such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), lithium triborate (LiB3O5: LBO), β-barium borate (β-BaB2O4), potassium titanyl phosphate (KTiOPO4: KTP), cesium lithium borate (CsLiB6O10: CLBO) have been developed.
Particularly, in the case of using lithium niobate crystal as nonlinear optical crystal, it is known that high conversion efficiency can be obtained by a large nonlinear optical constant. Since the construction of a device can be simplified, a quasi phase matched (QPM) wavelength conversion element formed using this crystal and poling technology is frequently used in devices having outputs of about 200 to 300 mW. Further, in a device capable of obtaining several W (Watts) output, nonlinear optical crystals such as LBO and KTP are used.
The above LBO crystal has a feature that breakdown and deteriorations due to fundamental wave and generated second harmonic are less likely to occur, whereas it is necessary to construct a resonator and arrange the crystal in the resonator in order to obtain high conversion efficiency, which necessitates a complicated device construction and precise adjustments since it has a small nonlinear optical constant. On the other hand, the KTP crystal has a larger nonlinear optical crystal than the LBO crystal and can obtain high conversion efficiency even without constructing a resonator, whereas it has a disadvantage that breakdown and deteriorations due to fundamental wave and generated second harmonic are likely to occur.
An example has been reported in which, by the crystal growth of lithium niobate and lithium tantalate by a method for doping additive in crystals as in patent literature 1 or approximating the crystal composition to an ideal composition (stoichiometric composition) as in patent literature 2, a refractive index change by light, i.e. optical damage as one of crystal deteriorations can be suppressed.
As described above, nonlinear optical crystals have advantages and disadvantages, and it is being studied to reduce the power density of fundamental wave per wavelength conversion element in order to suppress deteriorations by determining crystal to be used based on tradeoff between the advantages and disadvantages or by using a plurality of wavelength conversion elements as in patent literature 3. The construction of a wavelength converter disclosed in patent literature 3 is shown in FIG. 30.
As shown in FIG. 30, fundamental wave emitted from a fundamental wave light source 101 has second harmonic (green light) 105a split by a separation mirror 103a after being wavelength-converted by a wavelength converting portion 102a, the fundamental wave having passed through the separation mirror 103a has second harmonic (green light) 105b split by a separation mirror 103b after being wavelength-converted by a wavelength converting portion 102b and the remaining fundamental wave becomes residual fundamental wave 106. In this case, there were problems of having a complicated structure and doubling the number of parts since the number of wavelength conversion elements is increased.
The above-mentioned patent literature 1 and 2 disclose methods for doping magnesium oxide to avoid a phenomenon called optical damage. Patent literatures 4 to 7 and nonpatent literature 1 also describe methods for doping magnesium oxide to avoid a phenomenon called optical damage. For example, in the case of lithium niobate, it is generally well-known that this optical damage can be avoided if 5 mol or more of magnesium oxide is added. Besides, an example of realizing the generation of green light of 1.7 W by heating lithium niobate crystal doped with 5 mol of magnesium oxide to 140° C. is reported in nonpatent literature 2.
Specifically, the above-described optical damage is referring to a light induced refractive index changing phenomenon (photorefractive) in which electrons are excited by an optical electric field and refractive indices around a position, where a laser beam passed, change due to the electro-optic effect of a crystal. More specifically, the optical damage is caused only by green light (second harmonic) having a low output in the order of several hundreds mW if infrared light to become fundamental wave is converted into the green light (second harmonic) and is also caused even if magnesium oxide is not doped.
In order to suppress the photorefractive as one of the above crystal deteriorations, proposals have been made to control such that the absorption end of transmittance is located at a shorter wavelength by adding magnesium oxide or zinc oxide and/or to improve the transmittance of a general visible region (not transmittance when light having a specific wavelength is irradiated) in order to compensate for holes formed even after impurities forming an absorption peak in a crystal are maximally removed and electric charges generated by antisite defects where an element constituting a crystal is located on a site different from the original one.
However, crystal breakdown and deteriorations cannot be completely suppressed at present even if magnesium oxide is doped within the above range. Particularly, in the case of obtaining harmonic having a high output in the order of several W, crystal breakdown and deteriorations could not be suppressed.    Patent Literature 1: Japanese Patent No. 3261594    Patent Literature 2: Japanese Patent No. 3424125    Patent Literature 3:
Japanese Unexamined Patent Publication NO. H11-271823    Patent Literature 4: Japanese Patent No. 2720525    Patent Literature 5:
Japanese Unexamined Patent Publication NO. H06-242478    Patent Literature 6:
Japanese Unexamined Patent Publication NO. 2003-267799    Patent Literature 7:
Japanese Unexamined Patent Publication NO. 2003-267798    Nonpatent Literature 1:
Applied Physics letters, 44, 9, 847-849 (1984)    Nonpatent Literature 2:
Applied physics letters, 59, 21, 2657-2659 (1991)